The present invention relates to high impedance amplifiers having leakage current and a DC return for the leakage current. Specifically, the invention relates to amplifiers, such as field effect transistors (FET's), utilized in connection with a capacitive input signal source.
When high input impedance amplifiers are connected to capacitive input signal sources, a problem arises in the form of a DC-caused capacitive charge. The capacitive charge, if not prevented, interferes with the AC operation. In order to prevent the charge accumulation, a DC sink (return path) must be provided while still maintaining a high input impedance for AC signals. Also, it is desirable to maintain the input potential level, in the absence of an input signal, at a precisely defined value which is typically a value near zero (such as a few millivolts).
The problem typically arises when a capacitive bridge or other capacitive circuit is connected to provide an AC input signal to the gate of a field effect transistor (FET). The FET is connected as an amplifier with the gate reverse biased. The reverse bias on the gate causes a small leakage current which is present even in the absence of an input AC signal at the gate. Unless a DC return path is provided, the leakage current causes a charge to build up on the capacitive input. This charge adversely affects propagation of the AC signal.
One solution to charge build up problem is to connect a high-value resistor from the FET gate to ground or other sink potential. Such a resistor typically has a value of 1,000 megohms. These resistors are not satisfactory, however, since they are expensive, bulky and not readily incorporated in integrated circuit structures.
In view of the above background, there is a need for an improved amplifier structure having a leakage current return path, having a precise no-signal gate voltage, having a high AC input impedance and which is compatible with integrated circuit processing techniques.